mistral-executor: mount /var/lib/mistral

When containerizing mistral-executor, we need to mount /var/lib/mistral so
our operators can get the config-download logs when the undercloud is
containerized and config-download is used to deploy the overcloud.

To help our operators, we also create /var/lib/mistral/readme.txt so
they know where to find the config-download data.

Change-Id: I8d31d5fec2721c6e4f82b1ad2169a7635cb57600
Closes-Bug: #1749823

docker/services/mistral-executor.yaml

@@ -89,6 +89,9 @@ outputs:
             - path: /var/log/mistral
               owner: mistral:mistral
               recurse: true
+            - path: /var/lib/mistral
+              owner: mistral:mistral
+              recurse: true
       docker_config:
         step_4:
           mistral_executor:
@@ -110,6 +113,7 @@ outputs:
                   # exclude this on the overcloud for security reasons.
                   - /var/lib/config-data/nova/etc/nova:/etc/nova:ro
                   - /var/log/containers/mistral:/var/log/mistral
+                  - /var/lib/mistral:/var/lib/mistral
             environment:
               - KOLLA_CONFIG_STRATEGY=COPY_ALWAYS
       host_prep_tasks:

releasenotes/notes/mistral_container-23c4432aad469f30.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    When containerizing mistral-executor, we need to mount /var/lib/mistral so
+    our operators can get the config-download logs when the undercloud is
+    containerized and config-download is used to deploy the overcloud.